phpBB : Critical Error

Error creating new session

DEBUG MODE

SQL Error : 1062 Duplicate entry '-1' for key 'session_user_id'

INSERT INTO phpbb_sessions (session_id, session_user_id, session_start, session_time, session_ip, session_page, session_logged_in) VALUES ('4d33a1c0835e9d2b412509168470c6e3', -1, 1398072956, 1398072956, '3651502e', 13, 0)

Line : 158
File : /forum/includes/sessions.php